The monsoon rains that deluge much of South Asia every summer define the lives of the people who live and work there. But the strength and timing of the monsoon is very difficult to predict. Much previous research in this area has focused on the role of the Himalayas a key regulating factor, but new research by scientists from Massachusetts Institute of Technology (MIT) has now demonstrated that an interplay between atmospheric winds and the ocean waters south of India also has a major influence over its strength and timing.
Monsoons are generally understood to occur due to the increasing temperature difference between the land as it heats in summer and the comparatively cool ocean. This new research, published in the Journal of Climate, demonstrates that as the sun heats up the Indian subcontinent, it also causes a surge of strong winds that sweep across the Indian Ocean and up over the land. As these winds drive northward, they also push ocean waters southward. The researchers found that these south-flowing waters transport heat with them, cooling the ocean and thereby increasing the temperature gradient between the land and sea. ‘It’s this coupling of the trade winds with the ocean south of India which is critical in modulating the intensity and timing of the monsoon,’ explains John Marshall, professor of oceanography at MIT. The stronger the coupling between the winds and ocean, the wider the difference in land and sea temperature, and the stronger the intensity of the ensuing monsoon.
Marshall and his colleagues were able to demonstrate this effect by using a stripped-back, minimal model of the monsoon system, including only a landmass above the equator, the ocean, and a seasonal sun cycle. ‘What tends to happen is people build more and more complicated and sophisticated models so that it becomes very hard to tease the mechanisms apart and understand what’s at the heart of the machine,’ explains Marshall. ‘What we’ve tried to do is simplify things to the essence and get a minimalist model.’ In this case, less certainly appears to be more. Despite the fact that the model did not include the Himalayas, which established wisdom said was a necessary component, the researchers were still able to produce a simulated monsoon from the coupling effect of the ocean and the winds.
Ultimately, it is likely to be a combination of this coupling effect and the impact of the Himalaya which make the South Asian monsoon so strong. Marshall explains that the Himalaya block the Indian subcontinent from the impact of the polar regions and act as a heat source, contributing to a more extreme temperature gradient between land and sea. ‘But nevertheless, even if we take that process away and have a flat continent with no topographic detail, we can still get something that looks very like a monsoon,’ he says. The team now plan to improve their model which could serve to help climate scientists make future predictions about the monsoon.
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